1. Field of the Invention
The present invention relates to an apparatus and method of crystallizing an amorphous silicon film, and more particularly, to a sequential lateral solidification (SLS) apparatus and a crystallization method using the SLS apparatus.
2. Discussion of Related Art
Generally, polycrystalline silicon (p-Si) or amorphous silicon (a-Si) are materials used as the active layer of thin film transistor (TFTs) in liquid crystal display (LCD) devices. Since amorphous silicon (a-Si) can be deposited at a low temperature to form a thin film on a glass substrate, it is more widely used as an element of a switching device in liquid crystal display (LCD) devices. However, amorphous silicon (a-Si) has a difficulty in being employed in the large LCD devices because of its electrical characteristics.
In contrast to amorphous silicon, polycrystalline silicon provides faster display response time when used as an element of the TFT. Thus, polycrystalline silicon (p-Si) can be used in the large-sized LCD devices, laptop computers and wall television sets which need a larger field effect mobility of more than 30 cm2/Vs and a low leakage current.
Polycrystalline silicon is composed of crystal grains and grain boundaries. If the grains are larger and the grain boundaries are regularly distributed within the polycrystalline silicon, the field effect mobility becomes larger. In view of these grains and grain boundaries, a silicon crystallization method that produces large grains currently becomes an important issue. Accordingly, a sequential lateral solidification (SLS), which induces lateral growth of silicon grains to form single-crystal silicon film using laser energy, is proposed.
The SLS method of crystallizing an amorphous silicon layer uses the fact that silicon grains tend to grow vertically against the interface between liquid and solid silicon, and teaches that the amorphous silicon layer is crystallized by controlling the magnitude of laser energy and an irradiation of a moving laser beam to form silicon grains growing latterly up to a predetermined length. Therefore, to conduct the SLS method, an SLS apparatus is required as shown in FIG. 1.
FIG.1 is a schematic configuration of a sequential lateral solidification (SLS) apparatus according to a conventional art. In FIG. 1, the SLS apparatus 32 widely includes a laser generator 36, a mask 38, a condenser lens 40 and an objective lens 42. The laser generator 36 generates and emits a laser beam 34. The amount of the laser beam 34 emitted from the laser generator 36 is adjusted by an attenuator (not shown) that is in the path of the laser beam 34. The emitted laser beam 34 is applied to the condenser lens 40 such that the laser beam 34 is condensed after passing the condenser lens 40. The mask 38 includes a plurality of slits “A” through which the laser beam 34 passes and light absorptive areas “B” that absorb the laser beam 34. At this point, the width of each slit “A” defines a size of the grain when amorphous silicon is crystallized by a first laser irradiation. Furthermore, the distance between each slit defines a size of the lateral grain growth when the amorphous silicon film is crystallized by the SLS method. The objective lens 42 is arranged below the mask and scales down the shape of the laser beam having passed through the mask 38.
Further in FIG. 1, an X-Y stage 46 is arranged adjacent to the objective lens 42. The X-Y stage is movable in two orthogonal axial directions, such as x-axis and y-axis, and includes an x-axial direction drive unit for driving the x-axis stage and a y-axial direction drive unit for driving the y-axis stage. A substrate 44 is placed on the X-Y stage 46 in a location corresponding to the mask. Although not shown in FIG. 1, an amorphous silicon film is formed on the substrate 44, thereby defining a sample substrate. In this conventional configuration of the SLS apparatus, the laser generator 36 and the mask 38 are fixed in a corresponding position such that the mask 38 is not movable to crystallize the amorphous silicon film of the sample substrate 44. Thus, the X-Y stage should minutely move in an x-axial or y-axial direction to crystallize all the sample substrate 44.
A method of crystallizing an amorphous silicon film using the above-described SLS apparatus is explained hereinafter. A crystalline silicon film is generally formed by crystallizing the amorphous silicon film previously deposited on a substrate. The amorphous silicon film is deposited on the substrate using a chemical vapor deposition (CVD) method and includes a lot of hydrogen therein. The amorphous silicon film is thermal-treated to conduct the de-hydrogenation thereof, thereby reducing the amount of the hydrogen contained in the amorphous silicon film. The reason for the de-hydrogenation is to make a surface of the crystalline silicon film smooth. If the de-hydrogenation is not conducted, the surface of the crystalline silicon film becomes rough, and thus the electrical characteristics of the crystalline silicon film become degraded.
FIG. 2 is a plan view showing a substrate 44 having a partially-crystallized amorphous silicon film 52. When crystallizing the amorphous silicon film using the laser beam, it is difficult to crystallize a whole region of the amorphous silicon film at one time because the laser beam is restricted in its width, and the mask are also restricted in its size. Therefore, when the substrate is a large size, the mask should be arranged many times over the substrate, and thus, the crystallization processes are also repeated many times corresponding to each mask arrangement. In FIG. 2, an area “C” corresponding to one mask is defined as one block. At this point, the crystallization of the amorphous silicon within one block “C” is achieved by irradiating the laser beam several times.
The crystallization process of the amorphous silicon film will be explained as follows. FIGS. 3A to 3C are plan views showing one block of an amorphous silicon film in the crystallization process steps by using a conventional SLS apparatus. At this time, it is supposed that the mask has three slits therein.
FIG. 3A shows an initial step of crystallizing the amorphous silicon film when a first laser beam irradiation is carried out. As described in FIG. 1, the laser beam 34 emitted from the laser generator 36 passes through the mask 38 and irradiates one block of the amorphous silicon film 52 deposited on the sample substrate 44. At this time, the laser beam 34 is divided into three line beams by the slits “A”, and then these line beams irradiates regions “D”, “E” and “F” of the amorphous silicon film 52 in order to melt each region “D”, “E” or “F”. The energy density of the line beams is sufficient to induce complete melting of the amorphous silicon film. The liquid phase silicon begins to be crystallized at the interface 56 between the solid phase amorphous silicon and the liquid phase silicon. Namely, lateral grain growth of grains 58a proceeds from the un-melted regions adjacent to the fully-melted regions. The grain boundaries in directionally solidified silicon tends to form so as to always be perpendicular to the interface 56 between the solid phase amorphous silicon and the liquid phase silicon. As a result of the first laser beam irradiation, crystallized regions “D”, “E” and “F” are finally formed in one block corresponding to the mask 38 of FIG. 1, such that crystallized silicon grain regions “D”, “E” and “F” are induced.
FIG. 3B shows a step of crystallizing the amorphous silicon film when a second laser beam irradiation is carried out. After the first laser beam irradiation, the X-Y stage moves in a direction opposite to the lateral grain growth by a distance of several micrometers that is the same as or less than the length of the lateral growth. Then, the second laser beam irradiation is conducted. Therefore, the regions irradiated by the second laser beam are melted and then crystallized in the manner described in FIG. 3A. At this time, the silicon grains 58a grown by the first laser beam irradiation serve as seeds for the crystallization, and thus the lateral grain growth proceeds in the melted regions. Silicon grains 58b formed by the second laser beam irradiation continue to grow adjacent to the silicon grains 58a formed by the first laser beam irradiation.
Accordingly, by repeating the foregoing steps of melting and crystallizing the amorphous silicon, one block of the amorphous silicon film is crystallized to form grains 58c as shown in FIG. 3C. FIG. 3C shows one block of a crystalline silicon film resulted from lateral growth of grains to predetermined sizes.
Moreover, the above-mentioned crystallization processes conducted within one block are repeated through block by block in the amorphous silicon film. Therefore, the amorphous silicon film can be converted into the crystalline silicon film although it has a large size. However, the conventional SLS apparatus described above has some problems as follows.
First, in the crystallization process which uses the laser beam passing through the slits of the mask, the X-Y stage moves by a distance of several micrometers or millimeters to induce the lateral grain growth. However, it is very difficult to control the movement distance using the relatively large X-Y stage. Second, it takes 0.1 to 1 seconds that the X-Y stage moves and stops. However, if the substrate and the X-Y stage are large in size, it takes much more time to move the X-Y stage. Accordingly, the yield of crystallizing the amorphous silicon film is lowered.